1. Field of the Invention
The present invention generally relates to zoom lenses suitable for silver halide cameras, video cameras, electronic still cameras, and the like, and, more particularly, to a compact zoom lens that corrects well image shaking (or blur) at the time of vibration (or a tilting operation) of a zoom lens by moving lens groups or units of a part of an optical system or a part of the lens groups perpendicularly to an optical axis and by simultaneously maintaining high optical performance, and that has a relatively short back focus and a high variable magnification range.
2. Description of the Related Art
Hitherto, there have been proposed various optical systems (hereunder referred to as vibration-proof optical systems) for correcting image blur, which occurs during vibration of an imaging optical system, by driving optical members of the imaging optical system in some way, thereby stabilizing the images.
Generally, important requirements for such an optical system are that an optical element having a weight that is as light as possible is driven by a drive mechanism of a relatively simple configuration so as to make the driving amount as small much as possible, and that the optical performance thereof is favorably maintained over a vibration-proofing angle range that is as wide as possible.
Further, in a high magnification zoom lens having increasingly been developed in recent years, a large camera shake is apt to occur, especially, at a telephoto end thereof. Therefore, importance is attached to a vibration-proofing function. There have been proposed various zoom lenses each having a vibration-proofing function that have attempted to meet the aforementioned requirements and to have high magnification.
Incidentally, the vibration-proof optical systems include what is termed a shifting vibration-proof system adapted to displace all or part of lens groups perpendicularly to an optical axis, what is called a tilting vibration-proof system adapted to rotate all or part of lens groups around an axis of rotation that is perpendicular to an optical axis, and a variable angle prism system adapted to perform a vibration-proofing operation by providing a member, which is obtained by sandwiching relatively transparent liquid, such as silicon oil, between transparent members, in the vicinity of or inside an imaging optical system, and by inclining the transparent members with respect to an optical axis, and by utilizing a prism action. Additionally, various examples of the application of such a vibration-proof optical system to a zoom lens are known.
Among such vibration-proof optical systems, especially, the shifting vibration-proof optical system is widely adopted, because of the following facts. That is, although the shifting vibration-proof optical system is lower in flexibility in correcting aberration than the tilting vibration-proof system enabled to set an axis of rotation in a relatively free position, the shifting vibration-proof optical system has a relatively simple lens drive mechanism. Further, the shifting vibration-proof optical system is advantageous over the variable-angle prism-vibration-proof system in that the shifting vibration-proof optical system has high flexibility in correcting color aberration. Consequently, the shifting vibration-proof optical system has a relatively simple configuration and obtains relatively high optical performance.
Conventional vibration-proof zoom lenses each employing such a shifting vibration-proof system are disclosed in Japanese Patent Laid-Open No. 6-265827 (corresponding to U.S. Pat. No. 5,638,210), and Nos. 7-318865 and 8-82769 Official Gazettes.
Japanese Patent Laid-Open No. 6-265827 Official Gazette discloses the zoom lens, which comprises, in order from an object side, a first lens group of positive refractive power, a second lens group of positive refractive power, and a third lens group of negative refractive power. This zoom lens is configured so that, when performing zooming from a wide-angle end thereof to a telephoto end thereof, the spacing between the first and second lens groups increases, whereas the spacing between the second and third lens groups decreases. The second lens group is divided into a front group and a rear group. A shifting vibration-proof operation is performed by the rear group.
Japanese Patent Laid-Open No. 7-318865 Official Gazette discloses the zoom lens, which comprises, in succession from an object side, a first lens group of positive refractive power, a second lens group of negative refractive power, a third lens group of positive refractive power, a fourth lens group of positive refractive power, and a fifth lens group of negative refractive power. This zoom lens is configured so that, when performing zooming from a wide-angle end thereof to a telephoto end thereof, the spacing between the first and second lens groups increases, and the spacing between the second and third lens groups decreases, and the spacing between the third and fourth lens groups increases, and the spacing between the fourth and fifth lens groups decreases, and that at least the first and fifth lens groups are moved to the object side. A shifting vibration-proof operation is performed by the fourth lens group.
Japanese Patent Laid-Open No. 8-82769 Official Gazette discloses a zoom lens, which comprises, in order from an object side, a first lens group of positive refractive power, a second lens group of positive refractive power, a stop, and a third lens group of negative refractive power. This zoom lens is configured so that, when performing zooming from a wide-angle end thereof to a telephoto end thereof, the spacing between the first and second lens groups increases, whereas the spacing between the second and third lens groups decreases. A shifting vibration-proof operation is performed by the second lens group.
The zoom lens disclosed in Japanese Patent Laid-Open 6-265827 Official Gazette has a zoom ratio of about 2.6 and thus, has a low magnification level. If the magnification of the zoom lens is increased to 5 or so without changing the configuration thereof, the aberration at the time of vibration-proofing and the aberration in a normal or reference state deteriorate.
Moreover, the number of lenses of the vibration-proof lens groups is large, so that the weight of the zoom lens is relatively large. If various aberrations at the time of vibration-proofing, which are caused owing to an increase in magnification, are corrected, the number of the lenses of the lens groups increases. Consequently, a lens driving load at the time of vibration-proofing increases.
The zoom lens disclosed in Japanese Patent Laid-Open No. 7-318865 Official Gazette has a zoom ratio of about 3.3 and thus has a little higher magnification. Although this zoom lens has a relatively simple configuration of lenses of vibration-proof lens groups, the aberration at the time of vibration-proofing deteriorates.
The zoom lens, especially, a second embodiment disclosed in Japanese Patent Laid-Open No. 8-82769 Official Gazette has a zoom ratio of about 3.9 and thus has a relatively high magnification. In this zoom lens, the aberration at the time of vibration-proofing is relatively favorably corrected over a relatively wide range of vibration-proofing angles. However, the number of lenses of the vibration-proof lens groups is large. The lens driving load at the time of vibration-proofing is large. Further, when attempting to obtain a zoom ratio of about 5, the number of lenses of the zoom lens increases with the result that the size of the lens system increases. It is, thus, difficult to achieve both the correction of the aberration in the normal state and the correction of the aberration at the time of vibration-proofing by simultaneously maintaining the simple and compact configuration of this zoom lens.